To return to Blest, bless'd.

The American Webster's Collegiate Dictionary recognizes blest (although blessed is more common and pronounced blest); it is used by many Americans. It is neither dialect nor 'quaint' nor exclusively English nor exclusively poetic. Often abbreviated in the 19th C. as bless'd.
Blessèd (sometimes blesséd) is generally considered by Americans to be poetic or biblical.

Odd how "Sing all earth" got mis-heard as sin on earth, sent on earth, etc. Or did it? "Sin on Earth" appears in versions other that those mentioned above. There are many printed copies of this carol which, as far as we know, dates to the 16th century in Sussex. It seems The Copper Family does have it both ways in different versions.
Peter Bellamy sang an excellent rendition of this song, I don't have the recording so I don't know whether he used sing or sin.

Snuffy's Home Service lyrics are as good as any. The Grand Union Folk Club of Leicestershire has it with others on their website (with 'Sin on earth'). Carols